<template>
    <canvas id="canvas" ></canvas>
</template>

<script>

export default {
  data() {
    return {
    };
  },
  mounted() {
    this.initCanvas();
  },
  methods: {
    initCanvas() {
      var canvas = document.getElementById('canvas'),
        ctx = canvas.getContext('2d'),
        w = canvas.width = window.innerWidth,
        h = canvas.height = window.innerHeight,
        hue = 217,
        stars = [],
        count = 0,
        maxStars = 1400;
// Thanks @jackrugile for the performance tip! http://codepen.io/jackrugile/pen/BjBGoM
// Cache gradient
      var canvas2 = document.createElement('canvas'),
        ctx2 = canvas2.getContext('2d');
        canvas2.width = 100;
        canvas2.height = 100;
      var half = canvas2.width/2,
        gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
      gradient2.addColorStop(0.025, '#fff');
      gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
      gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
      gradient2.addColorStop(1, 'transparent');
      ctx2.fillStyle = gradient2;
      ctx2.beginPath();
      ctx2.arc(half, half, half, 0, Math.PI * 2);
      ctx2.fill();
// End cache
      function random(min, max) {
        if (arguments.length < 2) {
          max = min;
          min = 0;
        }
        if (min > max) {
          var hold = max;
          max = min;
          min = hold;
        }
        return Math.floor(Math.random() * (max - min + 1)) + min;
      }
      class Star {
        constructor() {
          this.orbitRadius = random(w / 2 - 50);
          this.radius = random(100, this.orbitRadius) / 10;
          this.orbitX = w / 2;
          this.orbitY = h / 2;
          this.timePassed = random(0, maxStars);
          this.speed = random(this.orbitRadius) / 900000;
          this.alpha = random(2, 10) / 10;
          count++;
          stars[count] = this;
        }
        draw() {
          var x = Math.sin(this.timePassed + 1) * this.orbitRadius + this.orbitX, y = Math.cos(this.timePassed) * this.orbitRadius / 2 + this.orbitY, twinkle = random(10);
          if (twinkle === 1 && this.alpha > 0) {
            this.alpha -= 0.05;
          } 
          else if (twinkle === 2 && this.alpha < 1) {
            this.alpha += 0.05;
          }
          ctx.globalAlpha = this.alpha;
          ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
          this.timePassed += this.speed;
        }
      }
      for (var i = 0; i < maxStars; i++) {
        new Star();
      }
      function animation() {
        ctx.globalCompositeOperation = 'source-over';
        ctx.globalAlpha = 0.8;
        ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 1)';
        ctx.fillRect(0, 0, w, h)
        ctx.globalCompositeOperation = 'lighter';
        for (var i = 1, l = stars.length; i < l; i++) {
          stars[i].draw();
        };  
        window.requestAnimationFrame(animation);    
      }
      animation();
    },
  },
};
</script>
<style scoped>
</style>
